type change = 
  | PATCH of string * string * string
  | ADD_FILE of string * string
  | ADD_DIR of string
  | DELETE_FILE of string
  | DELETE_DIR of string
  | RENAME_FILE of string * string
  | RENAME_DIR of string * string
  | ATTR_CLEAR of string * string
  | ATTR_SET of string * string * string

type edge = {
    old_revision : string ;
    change_set : change list ;
  }

type t = string * edge list